3. THE CLUSTER COMPONENTS

3.1. The morphology-density relation

It was probably Harlow Shapley
[413]
in 1926 the first to
explicitly refer to the different galaxy content of the Virgo and the
Coma cluster, Coma being dominated by
``spheroidal'' galaxy
types(7). However,
Shapley thought that
with increasing resolution many apparently featureless spheroidals
would turn out to be real spirals. Ten years after, in The Realm
of the Nebulæ, Hubble first hinted at the existence of a
morphology-density relation:

``There are some indications of a correlation between characteristic type
and compactness, the density of the cluster diminishing as the most frequent
type advances along the sequence of classification''

Hubble also noted the ``dominance of late typed among isolated
nebulæ in the general field''. The morphology-density relation was
immediately regarded as fundamental, to such a point that
Tombaugh [461],
in 1937, thought that a galaxy overdensity
dominated by spirals could not be a real cluster. In the same year,
Tombaugh noted that cluster ellipticals are more centrally
concentrated than cluster spirals. In 1942 Zwicky
[515] showed
that S0s in Virgo are distributed like ellipticals and unlike spirals.

In 1960 van den Bergh
[473]
first noted the existence of a
correlation between morphology and local galaxy density. By
examining the Ursa Major and Virgo clusters, he noted that

``there is some indication that the nebular population type
is related to the surface density of galaxies''

In those years, de Vaucouleurs
[131,
132]
(see also Abell
[12])
suggested that spirals and ellipticals in Virgo have different
distributions simply because they belong to different clusters. The
morphology-density relation was thus reduced to a mere projection
effect. An even more extreme view was taken by Neyman et
al. [323]
who maintained that the observed scarcity of spirals
in clusters with respect to the field could be understood as ``a
difference in the difficulty of observations''!

In 1965 an extreme case of morphological segregation was discovered.
Morgan & Lesh
[312]
noted that many clusters are centrally
dominated by ``supergiant galaxies'', that they called
cDs. These galaxies were shown to live in the densest cluster
environment only. Not only are cDs lacking in the field, but also in
poor clusters and groups. In fact, the central dominant galaxies of
the poor clusters classified by Morgan et al.
[311],
were later
shown to lack the characteristic extended envelope of cDs (Thuan &
Romanishin
[458]).

In the 70's the number of available galaxy redshifts increased
considerably, finally allowing a more reliable identification of
cluster members. Rood et al.
[385]
were then able to identify 16
spirals as members of the Coma cluster. The idea that rich clusters
are dominated by ellipticals and S0s was so firmly established that
Rood et al.'s was considered a ``striking'' result.

In 1974 Oemler
[331]
published his seminal paper The
systematic properties of clusters of galaxies. I. Photometry of 15
clusters. He noted that the morphological segregation in clusters
depends on the cluster content. The morphology-density relation was
interpreted as a relation between the morphological content of a
cluster and its compactness. Oemler constructed galaxy number density
profiles by type, and noticed a decreasing space density of spirals
towards the cluster centres, except in spiral-rich clusters. He also
noticed that spirals in cD-clusters have a shallower density
profile than ellipticals at large radii. However, he could not notice
any difference between the density profiles of S0s and ellipticals.

A year later, Gregory
[194]
showed that the fraction of spirals
indeed increases with the distance from the Coma cluster centre. He
wrote:

``The increase in relative numbers of spiral and irregular
galaxies with radial distance seems incontestable. The effect
is so strong as to be obvious to the eye on a casual inspection
of the Sky Survey''

This tendency for ellipticals to be more clustered than spirals was
shown by Davis & Geller
[121]
not to be restricted to
clusters. They applied the 2-point correlation function to the Uppsala
catalogue to show that morphological segregation exists on scales up
to 6 Mpc. Four years earlier, in 1972, Takase
[445] had already
pointed out a colour segregation of galaxies on the scale of the Local
Supercluster.

Figure 8. The variation of galaxy
population with the mean density
of clusters. Solid-line: ellipticals; dashed-line: S0s;
dotted-line: spirals. From Oemler (1977).

Figure 9. The fraction of E, S0, and S+I
galaxies as a function
of the logarithm of the projected density. The upper histogram
shows the number distribution of the galaxies over the bins of
projected density. From Dressler (1980a).

In 1977 Oemler
[333]
wrote that ``density is the physical significant
parameter in determining the galaxy population of a cluster.''
Figure 3 of his paper - here reproduced in Fig. 8 -
is qualitatively very similar to Figure 4 in the
1980 paper of Dressler
[142]
- here reproduced
in Fig. 9. Both figures show the fractional
variation of spirals, S0s and ellipticals as a function of the cluster
density. However, Oemler's density is the mean cluster density,
and Dressler's density is the local density around each
galaxy. Anyway, Oemler wrote (but did not show) that the same
morphology-density relation was also verified individually in clusters
dominated by early-type galaxies. The same year, even a spiral-rich
cluster (Abell 262) was found to display a ``striking''
morphological segregation (Moss & Dickens
[313]).

Times were mature for Alan Dressler's milestone paper, Galaxy
morphology in rich clusters: implications for the formation and
evolution of galaxies
[142],
published in 1980, and based on
the evergreen Catalog of morphological types in 55 rich clusters
of galaxies
[143].
Dressler pointed out that: i) regular as
well as irregular clusters display the same morphology-density
relation; ii) it is not the radial distance, but the local density,
the basic parameter which determines the morphology mix. Dressler's
conclusions are still controversial nowadays (see, e.g. Sanromà &
Salvador-Solé
[397]),
and it is possible that both global
cluster properties and the local galaxy environment may play a
role in determining the galaxy morphology
[453].

In the two following years, Bhavsar
[60] and
de Souza [126]
extended Dressler's morphology-density relation
into the low galaxy density regime, through the analysis of loose groups.

7 It was only in 1923 that Reynolds
[372] pointed
out the existence of many ``globular or ovoid'' nebulæ,
distinctly different from spirals.
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